Field of the Invention
The present invention relates generally to solenoids, and more particularly, to solenoids including a magnetic flux shunt member for providing a low reluctance magnetic flux path between an armature and a pole member, for example, of the solenoid as the armature is driven to and away from a full stroke position.
Electromagnetic actuators include a solenoid coil for moving an armature relative to a pole member or an end wall of a case of the actuator, for example, in carrying out a control function. When the armature is to be driven toward the pole member, initially, a large air gap will exist between opposing faces of the armature and the pole member. The air gap provides a high reluctance path for magnetic flux produced by the solenoid coil for driving the armature toward the pole member. The high reluctance results in a reduced magnetic force, particularly at the full stroke position for the armature. Consequently, a relatively large attractive force must be produced to move the armature toward the pole member. In known actuators, producing a greater force generally requires increasing the size of the solenoid coil, and resulting in a larger size for the solenoid package.
Both the response time of the actuator and the turn-on threshold are a function of the amount of attractive force produced by the device. The amount of force which can be generated by electromagnetic actuators is related to the relative sizes of the magnetic pole and the armature, the number of turns of solenoid coil and the current that is applied to the solenoid coil. The solenoid coil size generally determines the dimensions of the device because the solenoid coil is wound on the magnetic pole. Thus, methods of maximizing the attractive force generated by such devices are usually directed to optimizing the magnetic circuit of the device.
The operating efficiencies of actuators can be increased to some extent by improving the magnetic flux coupling between the magnetic pole piece and the armature. Arrangements for improving such magnetic flux coupling in proportional actuators are disclosed in copending U.S. patent application, Ser. No. 09/205,920 of James R. Ward and Derek Dahlgren, which was filed on Dec. 4, 1998, and which is assigned to the assignee of the present application. This application, Ser. No. 09/205,920, is incorporated herein by reference. The application discloses a proportional actuator which includes a saturation tip formed on the movable armature of the actuator for directing magnetic flux through a pole piece to the armature. The saturation tip bridges the air gap that exists between the opposing surfaces of the armature and the pole piece when the armature is spaced apart from the pole piece. The actuator includes a step-wound coil which provides a region of increased diameter for accommodating the saturation tip, allowing this working diameters of the armature and the pole piece to be increased for a given size actuator, with a corresponding increase in the attractive force produced by the magnetic circuit of the device actuator.
Maximizing attractive force is an important factor in latching solenoids. Most known latching solenoids use flat face to maximize the attractive force. Another technique for improving magnetic flux coupling, and thus attractive force, between a magnetic pole piece and armature of a latching solenoid is to provide a conical shape for the armature to concentrate the flux and thereby increase the attractive force. However, the use of a conical shape results in a smaller area for latching in latching solenoids. Thus, it would be desirable to minimize the effect of the air gap for magnetic flux to cross as the armature is being driven to the latched position.
The disadvantages and limitations of the background art discussed above are overcome by the present invention. With this invention, there is provided a solenoid including a member of a magnetic material including a pole face, an armature of a magnetic material and having an armature face opposing the pole face, and a coil assembly for positioning the armature relative to the member. The armature is adapted for movement relative to the member between first and second positions, with the armature face being spaced apart from the pole face, defining an air gap when the armature is in a first position and the armature face engaging the pole face when the armature is in a second position. The solenoid further includes a magnetic flux shunt structure located adjacent to the pole face. The magnetic flux shunt structure includes at least one magnetic shunt member of a magnetically permeable material which is configured and arranged to shunt at least a portion of the air gap when the armature is in the first position to provide a low reluctance magnetic flux path between the member and the armature. The armature is movable relative to the magnetic shunt member. In one embodiment, the magnetic flux shunt structure is configured as a ring. In another embodiment, the magnetic flux shunt structure is configured as a flat washer-like member.
In preferred embodiments, the shunt member is a free floating with respect to a magnetic pole member and the armature. That is, the shunt member is not fixed or attached to the magnetic pole member or to the armature. Rather, the shunt member is positioned in relationship with the armature by an element of a non-magnetic material which can be a bobbin of the solenoid winding or a separate element, for example.
The free floating shunt member eliminates the need for tight concentricity tolerances and the need for a bearing such as that which is required for a fixed shunt ring, such as that disclosed in U.S. patent application, Ser. No. 09/205,920, referenced above. The independent shunt member allows the flatness of the pole member to be easily maintained to facilitate the obtaining optimum latching forces. In the actuators disclosed in the above-reference application, in which the shunt ring can be part of the pole member, for example, optimum operation relies on the flatness at the bottom of a counter bore in either the pole member or the armature, which is much more difficult to maintain than when the pole surface is formed by machining a flat surface that does not include an outwardly projecting annular portion encompassing the pole surface. In addition, the separate shunt ring allows the pole member and/or the armature to be made of a material that is different from the material of the shunt member. For example, in one preferred embodiment, the shunt member is of a soft material which provides for improved pull-in force from the unlatched to the latched position. The armature and the pole member can be of hardened material which provides for improved residual latching forces in the latched position.
The magnetic flux shunt structure results in greater magnetic attractive force at relatively long strokes and tends to equalize the attractive force over the length of the stoke. Accordingly, for a given size package, a larger magnetic force is obtained for the solenoid including a magnetic flux shunt structure as compared to that produced for a comparably sized solenoid without the magnetic flux shunt structure. Alternatively, a comparable force can be provided using a lower level of current for energizing the solenoid winding, allowing the use a smaller package, as compared with a comparably sized solenoid that does not include a magnetic flux shunt structure. Moreover, because a larger force is provided, the solenoid can use a stiffer bias spring, if desired.
Another advantage provided by the magnetic flux shunt structure of the present invention is that the surface areas of the pole face and of the armature face can be maximized as compared to a comparably sized solenoid that does not include a flux shunt structure.
Yet another advantage provided by the magnetic flux shunt structure is minimization of the air gap for magnetic flux to cross as the armature is being driven from the first position to the second position.
In some preferred embodiments, the solenoid is a latching solenoid, the latching mechanism being either residual magnetism or a permanent magnet in the magnetic flux path. In such application, the solenoid can include a guide member for isolating the armature from the magnetic flux shunt structure. The guide member eliminates side loading which could cause the armature to become misaligned with the flat pole face, resulting in reduction of the latching force.
Other advantages and features of the invention, together with the organization and the manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.